Stratified Intense-Dose- Yttrium-90 Ibritumumab Tiuxetan (⁹⁰YIT ) with Bendamustine+Fludarabine Nonmyeloablative Conditioning for Allogeneic Stem Cell Transplantation in b-Cell Malignancies

Background: Nonmyeloablative allogeneic transplantation has the potential to induce long-term remissions in patients with relapsed lymphoma. However, a non-intense conditioning regimen enhances the risk of early relapse. Anti-CD20 antibody radioimmunotherapy (⁹⁰YIT) delivers radiation dose not only to the tumor cells that bind the antibody but also to inaccessible neighboring cells as a result of the cross-fire effect. Thus, we hypothesized that the addition of escalated ⁹⁰YIT dose to the recently published bendamustine+fludarabine conditioning regimen (Khouri et al. Blood 2014) would facilitate early cytoreduction in such patients and promote improved long-term disease control by the allogeneic graft. Organ doses from a ⁹⁰YIT weight-based activity prescription (mCi/kg) vary considerably, which justifies a dosimetry-based strategy for mCi/kg escalation.

Methods and patients: On days -22 and -14, rituximab was given at 250 mg/m2 preceding 111In ibritumumab and ⁹⁰YIT administration, respectively. Organ dosimetric assessment was performed based on serial 111In ibritumumab whole body scanning (0, 4, 24, 72 and 144 hours) , to select from among five ⁹⁰YIT mCi/kg prescriptions (0.5, 0.75, 1, 1.25 or 1.5) that would result in an estimated 10 - 12 Gy dose to the liver, lungs or kidneys. Organ dose was corrected for patient-specific mass, based on a CT volume estimate times 1.03 g/cc for liver and kidneys, and a variable specific gravity for lungs (Simon, J Clin Monit Comput, 2000). Bendamustine 130 mg/m² plus 30 mg/m² of fludarabine IV were given daily on days -5 to - 3 prior to transplantation. Tacrolimus and mini-methotrexate (Mycophenolate mofetil in case of cord blood transplantation) were used for GVHD prophylaxis. In addition, thymoglobulin 1 mg/kg IV was given on days -2, and -1 in patients receiving an unrelated donor transplant.

Results: Twenty patients were studied. The median age was 58 years (range, 37-71). Lymphoma histologies included: indolent (n=8, 40%), diffuse large cell (n=6; 30%), double-hit (n=2; 10%) and mantle cell (n= 4, 20%). The median number of prior chemotherapies received was 4 (range, 2-7). At study entry, 8 patients (40%) were in complete remission following salvage therapy, 7 (35%) were in partial response, and 5 (25%) had refractory disease. Six of 16 (37.5%) patients tested were PET+. Dosimetry: The most exposed organ was either liver (16 patients) or lungs (4 patients). The distribution among the five ⁹⁰YIT mCi/kg prescriptions (smallest to largest) was 2, 4, 12, 1 and 1, with a mean of 0.94 ± 0.23 mCi/kg. If all twenty patients were treated at 1 mCi/kg (the most common prescription), the 20 Gy limit employed for ⁹⁰YIT clinical trials prior to approval would have been exceeded in only one patient for the liver (22.9 Gy) or lungs (20.9 Gy). The maximum liver and lung doses at 0.75 mCi/kg would have been 17.2 and 15.7 Gy, respectively. Transplant outcomes: Fifteen patients (75%) received their transplants from unrelated donors (including 1 mismatched and 2 cord blood), and only 5 (25%) from HLA-compatible siblings. The median number of CD34+ cells infused was 6.2 × 10⁶/kg. Neutrophil counts recovered to > 0.5 × 10⁹/L after a median of 12 days (range, 0-24 days). Platelet counts recovered to > 20 × 10⁹/L after a median of 19 days (range, 9-30 days). By day 30, median donor myeloid and T-cells were 100% (range, 98-100). The cumulative incidence of acute grade 2-4 GVHD and chronic extensive GVHD were 25% (5% for acute grade 3-4) and 32%, respectively. Treatment-related mortality (TRM) rates at day 100 and 1 year after transplantation were 0% and 10%, respectively. The 2 cord blood transplants engrafted with 100% donor cells and none had GVHD. With a median follow-up duration of 14 months (range, 3-34 months), the overall survival and progression-free survival rates were 85% and 70%, respectively. No significant difference in survival or TRM could be detected by age, donor type, histology, disease status, PET status or number of prior therapies.

Conclusions: Our results indicate that dose-intense ⁹⁰YIT combined with fludarabine and bendamustine is a well-tolerated nonmyeloablative allogeneic conditioning for lymphoid malignancies, with promising results of engraftment, GVHD and survival. Our stratified ⁹⁰YIT prescription results suggest that future studies with a fixed dose of 1 mCi/kg level without dosimetry would have an acceptable radiation risk to vital organs in this setting.